Self-twisted yarns are yarns that comprise two or more strands that have areas of twist in what is known as the z-direction and areas of twist in what is known as the s-direction (that is, the strands have alternating areas of twist in opposing directions). The twisted areas in the strands are each separated by non-twisted areas. Torque is trapped in the twisted strands and is otherwise known as residual twist. When two or more twisted strands are brought together, the torque, or residual twist, causes the strands to twist around each other naturally, without interference, to form a self-twisted yarn.
The different fabrics and purposes for which self-twisted yarns are used may require different yarns having a specific yarn structure suitable for that purpose. For example, next-to-skin fabrics used in vests generally require lightweight and soft yarns, fabrics to be used in socks require yarns that are capable of wicking away moisture, and fabrics that are used in outer layers of clothing, such as trousers, require yarns of sufficient strength that the fabric is long-wearing.
Further information regarding self-twisted yarns is found in the book “Structural Mechanics of Fires, Yarns and Fabrics”, by J. W. S. Hearle, P. Grosberg, and S. Backer, John Wiley and Sons Inc, USA, 1969, page 139; and also in the book “The Mechanics of Wool Structures”, by R. Postle, G. A. Carnaby, and S. de Jong, Ellis Horwood Ltd, England, 1988, page 131.
In producing a yarn formed of staple fibres or predominantly of staple fibres, such as wool, cotton, synthetic staple fibres, or a mixture of such fibres, a number of slivers may, typically after drafting, be passed through a twisting stage, which comprises reciprocating rotating rollers (twist rollers), which move from side to side as the slivers pass between the rollers, thereby imparting a twist to the strands. After exiting the twist rollers, the strands are brought together to twist naturally with each other to form a multi-ply yarn. Apparatus or machines for so producing a yarn are disclosed in Australian patent specifications 51009/64, 9432/66, 26099/67, and 25258/71.
New Zealand patent 336048 discloses a method for producing a yarn comprising three or more slivers, or ends, in which the three slivers are passed between reciprocating twist rollers and then one or more of the slivers is passed over a path of a different length before the slivers are brought together. Rather than all of the slivers or ends passing through the twisting stage together and then being twisted naturally together, the twist in one or more of the slivers or ends is staggered or out of phase relative to the twist in the other slivers.
PCT Patent Application No. PCT/NZ2003/000253 (published as WO 2004/044290) discloses an apparatus for producing a yarn comprising a plurality of twisted strands, which enables aspects of the twist profile imparted to the yarn to be controllably varied by using a control system to control the rotational speed of the twist rollers and/or the extent of transverse movement of the twist rollers, and thus influences the properties of the yarn or fabric or knitted or woven products formed from the yarn.
These prior art apparatus and methods allow for some forms of yarns having different yarn structures to be produced. However, one major problem with the yarn produced by these prior art apparatus and methods is that the yarn structure is inconsistent both along the length of a yarn produced and between different runs of yarn produced. Inconsistent yarn structures are undesirable and result in lower grade and inconsistent quality fabrics being produced when using such yarns. The inventors of this invention have found that this inconsistent yarn structure can be due to fluctuations in tension imparted on the strands of the yarn during self-twisting of the yarn. The prior art apparatus and methods have the disadvantage that they are unable to control the tension imparted on the twisted strands of yarn after the strands exit the twisting stage.
The inventors have identified a way of alleviating the inconsistencies in yarn structure by controlling the tension imparted on twisted strands as the strands self-twist together to form a yarn. By controlling the tension imparted on the twisted strands, it is not only possible to create yarns having a substantially consistent yarn structure, but is also possible to produce a greater variety of yarns having certain yarn structures/twist profiles according to the use for which the yarns will be employed. Therefore, it is an object of the invention to provide apparatus and a method for improved control of the yarn structures of yarns produced, so that purpose specific yarns having a consistent yarn structure can be manufactured, or to at least provide a useful choice.